The present invention relates generally to the field of methods of and system for capturing fingerprint images, and more particularly to a method of and system for compensating for injection gradient in a capacitive fingerprint sensing circuit array.
Fingerprint recognition has been suggested for use in many security applications, such as controlling access to buildings, computers, or the like. Fingerprint recognition systems enable a user to access the controlled facility without having to provide a device such as a keypad or card reader, and without having the user memorize a password, or other personal identification number, or carry a card key.
An important element of a fingerprint recognition system is a sensing device. An example of a sensing device is the TouchChip (TM) Silicon Fingerprint Sensor, which is available from STMicroelectronics, Inc. The TouchChip uses an active pixel array based upon a capacitive feedback sensing circuit. The array comprises 360 rows and 256 columns of cells that represent pixels. Each pixel cell contains a high-gain amplifier connected to two adjacent top metal plates separated from the skin surface by an ultra-hard protective coating. The amplifier input is connected to one of the top metal plates and the inverter output is connected to the other top metal plate. The cell provides a charge integrator whose feedback capacitance is the effective capacitance between the two top metal plates.
When a finger is placed on the sensor, the surface of the skin over a pixel cell acts as a third plate separated from the two adjacent plates by a dielectric layer composed of air. Because fingerprint valleys will be farther from the sensor surface than fingerprint ridges, pixel cells beneath valleys will have more distance between their top metal plates and the skin surface than pixel cells under ridges. The thickness of the dielectric layer modulates the capacitive coupling between the top metal plates of the pixel cell so that top metal plates under valleys will exhibit different effective capacitance than top plates under ridges.
The pixel cell works in three phases. The first phase is a RESET, in which the input and output of the charge integrator pixel cells through a CMOS reset transistor driven by a reset signal. The second phase disconnects the output and input plates by asserting to ground the reset signal that drives the reset transistor. By opening the reset transistor switch, channel charge is injected into both the input and output plates. During the third phase, a fixed charge is applied to the charge integrator input, which causes an output voltage swing inversely proportional to the feedback capacitance, which is the effective capacitance between the top metal plates. Since the distance between the skin and a pixel cell changes the effective feedback capacitance of the charge integrator, the output of pixel cells under ridges will be different from the output of pixel cells under valleys.
Returning to the concept of injection, when the reset transistor is active or on, there is a conduction path channel that extends from the source to the drain of the reset transistor. When the gate voltage decreases, to switch off the reset transistor, mobile carriers are drained out of the channel through both the source and drain ends. The amount of channel charge that is injected into the input offsets the output of the charge integrator modifying the background of the image. The amount of charge injection depends on several factors. These factors include the slope of the signal applied on the gate, the input/output capacitance ratio, and the size of the reset transistor itself.
The reset signal is driven by a common buffer located at the top of the array. As the distance between the common buffer and the local pixel cell increases, the slope of the reset signal becomes lower due to the RC load of the line. The change in the slope of the reset signal over the length of the line causes different charge injection as the distance from the output of the buffer increases. At the top of the array, near the reset buffer, the amount of charge injected brings the pixel to its maximum saturated level giving a very dark image. Closer to the bottom on the array, the amount of injected charge decreases making the image lighter. The lighter image near the bottom of the array may cause certain fingerprint features to be less easily distinguishable, thereby resulting in inaccurate fingerprint recognition. The injection gradient problem is particularly acute in large arrays where the distance from the reset buffer to the bottom of the array is significant.
The present invention provides a capacitive fingerprint sensor and a method of capturing a fingerprint that compensates for injection gradient. The fingerrint sensor of the present invention includes an array comprising a plurality of capacitive pixel cells. Each of the capacitive pixel cells includes a pair of metal plates, a charge integrator, and a reset transistor, among other things. The reset transistor source is connected to one of the metal plates and its drain is connected to the other of the metal plates. The fingerprint sensor of the present invention includes a plurality of reset signal regenerators, there being at least one regenerator associated with each capacitive pixel cell. In the preferred embodiment, the regenerator includes an inverter. Each local inverter receives the input from the global reset line driven by the common buffer and regenerates the slope of the reset signal that controls directly the CMOS switch. The fingerprint sensor of the present invention includes a common reset buffer for generating a reset signal. The reset buffer includes an output that, through a common line, is connected to the input of each local inverter. When the reset buffer generates a reset signal, each inverter regenerates the reset signal slope at each reset transistor of each capacitive pixel cell, thereby equalizing the reset signal slope and so the injection, for every pixel cell. This, therefore, compensates for injection gradient along the column of the array of capacitive pixel cells.